1. Field of the Invention
The invention pertains to radiometric devices for seeking targets against a background of high noise temperature such as that of the earth's surface and more particularly relates to radiometric systems adapted while searching to detect special characteristics of such targets, even though immersed in noisy environments, by processing received signals representative of those special characteristics and by their recognition by processing systems which include adjacent-target-range discrimination apparatus.
2. Description of the Prior Art
Radiometric target searching and tracking systems, both passive and active, are known in the art and are generally characterized by several faults making them not totally satisfactory for use in high noise level environments, such as in detecting relatively small metal objects like navigation markers or vehicles in a background of ground clutter. In particular, prior devices employing only conventional types of scanning antenna systems during search generally have a limited field of view so that search of an area of reasonable size requires an unreasonable amount of time. Furthermore, in many prior systems, the wave form emerging from the conventional demodulator system employed varies widely with the location of the target with respect to the antenna scan axis and with other factors especially including the nature and level of the background noise present. Generally, the received wave forms have no invariant characteristic by which the presence and nature of a potential target can readily be distinguished preparatory to initiating acquisition and automatic tracking of a selected target.
An advanced multiple mode object seeker system is the subject of the Lazarchik et al U.S. Pat. No. 3,921,169 for a "Multiple Mode Radiometric System with Range Detection Capability" issued Nov. 18, 1975 and assigned to Sperry Rand Corporation. This microwave radiometric system features operation selectively in passive and active modes, providing in the active mode incoherent frequency-modulated continuous wave illumination for augmenting the apparent temperature of the target and for improving the measured radiometric temperature contrast between the target and its spatial background, consequently increasing the maximum distance at which such a target may be detected. Composite noise and triangular wave frequency modulation features of a closed-loop control system cooperate to yield a constant receiver beat frequency in a system in which target range is easily derived. The Lazarchik et al system will be recognized as an interesting continuous-wave frequency-modulation radiometric system with selective passive and active modes, but it will also be readily apparent that it provides no means for reliably recognizing a target embedded in a noisy background or for cooperatively acquiring and automatically tracking such a target. In addition, the Lazarchik et al system does not assess to solve the problem of operating in a situation in which an airborne craft is flown in an essentially horizontal path with the large increase in background clutter and reduction in operating range that accompanies operation of the antenna pattern at the near grazing incident angle with respect to the earth's surface.
The problems associated with positive recognition of a navigation marker or parked vehicle, for example, at a reasonably large range are connected with the usual constraints found in object detection systems of severe background clutter, variable weather clutter and attenuation, and receiver-generated noise. The problems are further complicated with targets that are inherently stationary such as navigation markers and many kinds of military targets. Where there is no target motion with respect to the terrain, the problems are even more serious, because no improvement in signal-to-clutter ratio can result by normal methods of Doppler processing. Accordingly, there is a need not filled by the prior art to achieve maximum spatial resolution of target detection by the application of the principles of signal processing. In the present invention the problems of the prior art are overcome by apparatus wherein spatial resolution is enhanced by employing a predetermined degree of resolution in the range dimension.